Experiments for the coming year will concentrate on elucidating the spontaneously occurring age-induced changes in the CNS-Pituitary-Gonadal Axis of the male rat. Pituitary and serum gonadotropin, prolactin, TSH and GH concentration will be measured by radioimmunoassays. Serum testosterone and dihydrotestosterone concentrations will be measured by solid phase radioimmunoassay. Pituitary and serum samples will be collected every two months until the age of 28 months. Endocrine changes will be correlated with structural changes observed histologically in the testes and accessory structures. Particular attention will be paid to any potential changes in the 24 hr pattern in the secretion of the above hormones. In female rats, an attempt will be made to elucidate any possible changes in the sensitivity of various components of the CNS-P-G axis to stimulatory or inhibitory signals from other parts of the system. Trophic or inhibitory stimuli will be tested by both in vivo and in vitro approaches. Finally, experiments will be initiated to investigate any possible changes in receptor activity to trophic hormones at the gonadal level and to gonadal steroids at hypothalamic and pituitary sites. These studies will be conducted in both male and female animals.